This invention relates to an apparatus for nuclear magnetic resonance imaging, and particularly to a method and apparatus for imaging moving liquid.
The technique related to this invention is discussed in publication: IEEE TRANSACTIONS ON MEDICAL IMAGAING, Vol. MI-5, No. 3, September 1986, pp. 140-151, and it will be briefed in the following. The operational procedures of imaging blood vessels including a moving part by the nuclear magnetic resonance imaging apparatus will be explained in connection with FIG. 3. In the operation, a pair of pictures called "sensitive picture" and "insensitive picture" are taken, and the latter is subtracted from the former to produce a picture of a blood vessel running system.
Initially, a sequence (a), (b), (c) and (d) is followed to produce a sensitive picture. On assumption that a blood stream has a dominant direction coincident with the x direction, the imaging procedures are as follows.
(1) A magnetic resonating RF pulse 301 is applied and, at the same time, a z-direction gradient magnetic field 302 is applied so that magnetization on a specific slice plane is excited. Another z-direction gradient magnetic field 303 is applied so as to align the phase of the excited magnetization.
(2) Next, a y-direction gradient magnetic field 304 is applied for implementing phase encoding.
(b 3) At the same time, x-direction gradient magnetic fields 305 and 306 are applied sequentially in the direction coincident with the dominant blood flow direction, and the MR signal 307 is measured during the application of the magnetic field 306.
(4) The measured MR signal 307 is rendered the Fourier transformation process, and a picture is reproduced.
The x, y and z directions are the axes of a 3-dimensional orthogonal coordinate system established in the space where a body under test is placed.
Next, a sequence (a), (b), (c) and (f) of FIG. 3 is followed to produce an insensitive picture. The above items (1) and (2) are equally applied.
Next, as item (b 3), a series of x-direction gradient magnetic fields 308, 309 and 310 are applied, and the MR signal 307 is measured during the application of the magnetic field 301. The measured MR signal 307 is operated by the Fourier transformation process, and a picture is reproduced.
The following explains the principle of producing a picture of the blood vessel running system by subtracting the sensitive picture from the insensitive picture obtained as described above.
In FIG. 3, phase rotations of magnetization in the body under test caused by the applications of the x-direction gradient magnetic field 305 and magnetic field 306 in its former half period T are called "first phase" and "second phase", respectively. In a quiescent part, the sum of the first phase and second phase is zero, and the phases are consistent at the center time of echo. This is a signal for the quiescent part. In a blood stream part, magnetization moves at a velocity within the gradient magnetic field, and in this case the sum of the first phase and second phase does not become zero, but results in a variable value depending on the blood flow velocity v. Assuming that the v is constant at all times and it is in the x direction, the sum .phi. of the first phase and second phase is formulated as follows. ##EQU1## where .gamma. is a constant, G is the gradient of the x-direction magnetic field, and T is the application time length of the x-direction gradient magnetic field.
Generally, the blood flow velocity v is variable along the radial direction of blood vessel, i.e., slicing direction, and therefore the phase .phi., which is dependent on the blood flow velocity v, also varies along the slicing direction. Consequently, measured signals cancel out each other, resulting in a small amplitude of signals. From this viewpoint, the blood vessel running system is not imaged on the basis of only a sensitive picture.
In the insensitive sequence, the phase rotation of magnetization caused by the application of the serial x-direction gradient magnetic field 308 and magnetic field 309 in its former half period T is called "third phase", and the phase rotation of magnetization caused by the application of the x-direction gradient magnetic field 309 in its latter half period T and magnetic field 310 in its former half period T is called "fourth phase". The sum of the third and fourth phases in a quiescent part is zero, as in the case of the sensitive sequence mentioned above. In a part of blood stream, if the blood flow velocity v is constant at all times and is in the x direction, the sum .phi.' of the third and fourth phases is also zero. The reason is that the third phase .phi..sub.3 and fourth phase .phi..sub.4 are expressed as follows through the same mathematical treatment as for the above case of .phi.: EQU .phi..sub.3 =-2.pi..gamma.T.sup.2 Gv EQU .phi..sub.4 =2.pi..gamma.T.sup.2 Gv EQU And, .phi.'=.phi..sub.3 +.phi..sub.4 =0
Accordingly, the variability of phase .phi.' due to flow velocities does not occur, and the signal for a blood stream part is produced when phases are consistent again at the center time of echo. Consequently, blood vessels appear in the insensitive picture. By subtracting the sensitive picture from the insensitive picture, the quiescent part is cancelled out and only the blood vessel running system is imaged.
Another related technique includes the one in which the angle of magnetization (of a quiescent part) fallen by the magnetic resonating RF pulse is made 360.degree.. Although magnetization of quiescent part is not excited, magnetization of a blood stream part is not fallen completely by 360.degree. due to the flow, and therefore it has a lateral magnetization. After that, by measuring the signal through the induction of echo by following the sequence shown by (a), (b), (c) and (f) in FIG. 3, only the signal for the blood stream part can be produced. The sequence is identical to the case of producing a sensitive picture, except that the magnetic resonating RF pulse 301 of (a) in FIG. 3 is replaced with a 360.degree.-falling RF pulse. As a result, the blood vessel running system is imaged from a single insensitive picture.
However, the foregoing related techniques of blood vessel imaging do not comprehend the nullification of phase difference, at the center time of echo at which the measured signal exhibits a maximum value, between the resonance signal for a part having a flow velocity and an expected resonance signal for the same part with the assumption of quiescence (this phase difference will be termed "flow velocity phase"). Therefore, due to various blood flow velocities in reality, there is a great disparty in flow velocity phases as the time point is moved forward and backward from the center time of echo, and therefore even an insensitive picture cannot extract blood vessels clearly.